The present invention relates to drinking vessels for holding liquids, with particular applicability to drinking cups.
In certain environments, drinking cups are very likely to tip over causing spillage of their contents. Such environments extend to adult usage, such as inside moving vehicles, as well to child or infant usage, such as on the tray of a high chair or children's dining tables in general. As a result, various designs for self-righting cups, i.e., cups which will return to their upright position when tilted, have been developed.
A common way of achieving this effect is to provide ballast in the base of the cup. This has also been combined with providing the cup with a rounded bottom. Such structures are generally unsatisfactory, however, since the angle through which the cups can be tilted and then returned to the upright position is fairly limited, and round-bottom vessels tend to rock at the slightest jostling.
The present invention provides a self-righting vessel of unusual stability, which neither rocks nor relies on a weighted base. The self-righting capability is imparted to the vessel by the shape of the vessel sides, whose vertical cross section is an involute defined by a hypothetical circle located inside the cup.
The term "involute" is used here in accordance with its common accepted definition, i.e., a curve traced by a point of a perfectly flexible inextensible thread kept taught as it is wound upon or unwound from another curve (in this case, a circle). The involute thus spirals in towards the circle, although as used herein it is truncated prior to meeting the circle. The most extended portion of the involute is at or in proximity to the rim at the mouth of the vessel, while the least extended point is at or adjacent to the bottom of the vessel. The bottom of the vessel connecting the sides is recessed forming a ring which rests flat on a table surface.
In various embodiments of the invention, the cup itself is a body of revolution about a vertical centerline, and the various circles defining the involutes of the sides have a common center located on the centerline, and revolve around the centerline to form a hypothetical sphere. The circles and sphere are of course imaginary and are referred to herein only for purposes of establishing the shape and location of the involutes forming the profiles of the sides of the vessel. They are not part of the actual structure of the vessel.
Also in various embodiments of the invention, the center of gravity of the vessel once it is tilted away from the upright position is offset from the reaction force vector resulting from contact of the vessel with the surface on which it is resting. As will be seen from the description which follows, tilting the vessel to shift the contact point from the bottom to one of the involutes shifts the reaction force vector to a position tangent to the circle defining the involute. Placing the center of gravity of the cup inside or below the circle assures that the offset mentioned above will be maintained until the cup rights itself. The offset will always be in a direction which urges the cup back to its upright position.
In further preferred embodiments the ratio of the cup diameter at the most extended point on the involute to the diameter of the circle defining the involute is from about 2.0 to about 7.0 with about 3.0 to about 4.0 preferred. Further dimensions are preferred as placing the center of gravity of the cup inside the involute circle and above the center of the circle. In still further preferred embodiments, the cup includes a removable lid or cover, containing a spout and a vent hole. When in place, the lid further assures that the liquids inside the vessel will remain in a location whereby the overall center of gravity (including the vessel, lid and liquid contents) at any angle of tilt will be offset from the reaction force vector at the contact point in the direction tending to urge the vessel back to its upright position.
Further embodiments of the invention will be apparent from the description which follows.